Frankie Valli made it seem so easy, spinning out those distinctive falsetto notes in hit songs as lead singer for the Four Seasons, starting 51 years ago with "Sherry." But what came naturally to Valli takes honing for the guys who have to approximate his vocal cords in the hit bio-musical "Jersey Boys."

The success of the show, which returns to Baltimore next week, depends to a large extent on filling that central role persuasively.

"The score is so difficult," says Katie Agresta, the seasoned New York-based vocal coach who has worked with the likes of Jon Bon Jovi and Cyndi Lauper. "Typically, a lead in a Broadway musical has maybe five songs. In 'Jersey Boys,' it's 27. And you have to act. That's why we developed what we call 'Frankie Camp.'"

The "camp" is held periodically in New York for actors who have shown potential. In addition to intensive sessions on acting and choreography, the camp focuses on the sound that made a short, Newark-born Italian-American one of the most instantly recognizable singers in pop music.

"Frankie had an amazing instrument with an amazing falsetto," says "Jersey Boys" music director Ron Melrose. "Usually, a falsetto is soft and choirboy-ish, but there was a gritty, street sound to his."

On the first day of camp, Agresta zeroes in on that technique.

"Frankie flipped into [falsetto] like ice skating in and out of a park," she says. "He just did it. I work with the guys on learning how to do that smoothly."

Agresta also gives the singers yoga exercises and diet suggestions to keep the voice in shape.

The goal is to cultivate security and stamina — one Frankie performs six shows a week, an alternate Frankie two. Another two cast members understudy the role, which gives each production the security of having four possible Frankies at a time.

"If there is a young man out there under five-foot-nine who looks Italian and has something of a falsetto, [the producers] know about them already," says Hayden Milanes, the alternate Frankie in the cast coming to Baltimore's Hippodrome.

Milanes, 28, had only joked around with falsetto before seeing John Lloyd Young, the first Broadway Frankie, singing on the Tony Awards. "That inspired me to get a voice teacher," he says.

Nick Cosgrove, the main Frankie for the touring cast, felt confident when he attend the camp two years ago at the age of 23.

"This has been my dream show since I was a senior in high school," he says "I had been a boy soprano, and I still had access to that part of my voice when I auditioned."

Every show day, Cosgrove still uses a 26-minute tape of vocal exercises Agresta suggested. Milanes likewise adheres to her advice, including the post-performance "warm down" she insists on — singing in descending order the same scales used to warm up, decreasing the volume as you go.

"Doing this show is sort of like running a marathon," Milanes says. "You wouldn't run a marathon and then say, 'Awesome, let's go eat cake.' You'd warm down."

After years of helping to prepare Frankies for a good run in "Jersey Boys," Agresta remains enthusiastic.

"My life is filled with young men who look alike and sing the same music," she says with a laugh. "They've enriched my life tremendously. They deserve all the success they have had."

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